It's a font resolution problem, serif fonts from a PDF generated by a modern LaTeX compiler like XeLaTeX are still more readable. If you zoom in, you'll see the

It's a font resolution problem, serif fonts from a PDF generated by a modern LaTeX compiler like XeLaTeX are still more readable. If you zoom in, you'll see the fonts are bitmaps so it's low resolution with no anti-aliasing. Apparently, the original METAFONT Computer Modern is rasterized after rendering, meanwhile today most people have switched to its Type-I equivalent, which remains vectorized in PDFs.